Fluctuating fuel prices, unabated energy sustainability concerns, and waste energy byproducts generated in industry have created the opportunity to develop fuel flexible combustion systems. A combustion system's capability to handle multiple liquid fuels depends on the fuel injector. Most combustion applications have limited fuel flexibility mainly because of the strong dependence of the injector performance on physical and chemical properties of the fuel. Thus, an ideal fuel injector would perform robustly with minimal dependence on fuel properties. The most common fuel injection techniques are: pressure driven as in direct injection systems, and kinetic energy driven as in twin-fluid atomizers. Less commonly used techniques include centrifugal energy driven atomization as in rotating discs, and effervescent, flashing, electrostatic, vibratory, and ultrasonic atomizers.
Twin-fluid injectors utilize kinetic energy provided by a gas introduced in the injector system, mainly for the purpose of enhancing atomization of the liquid fuel. An air-blast (AB) injector is a typical example of a twin fluid atomizer. In AB atomization, atomizing air and liquid are supplied separately to the injector. Air is delivered and swirled on the outer periphery of the injected liquid fuel at a relatively large velocity to break up the ejected fuel and to disperse the resulting spray in the combustion zone. The primary driving force of liquid break up and droplet formation is by the shear forces formed because of the high relative velocities between the two phases. However, a major shortcoming of this technique is that in highly viscous liquids such as glycerol or straight vegetable oils, or other alternative and opportunity fuels, shear layer instabilities are suppressed, giving rise to less effective droplet break up or larger droplet diameters in the spray.
Another twin fluid injector is an effervescent atomizer (EA). In EA, a pressurized gas is injected into the bulk liquid fuel inside an atomizer body, upstream of a nozzle orifice from which the fuel-air mixture is ejected into the combustion zone. Bubbles formed by the injected gas are then expanded rapidly when the two-phase mixture is exposed to a low pressure zone at the orifice exit, breaking up the liquid into droplets. EA is reported to produce a spray with very fine droplets. However, this method has known drawbacks in that the spray angle is usually narrow and atomizing air must be pressurized to the fuel supply pressure. This pressurization can be difficult to accomplish and might require large amounts of power. In addition, the spray produced can exhibit undesirable unsteadiness related to two-phase mixing flow processes in the channel downstream of the mixing chamber.
Accordingly, an improved fuel-flexible combustion system is needed that yields low emissions, requires low power, is suitable for alternate liquid fuels including highly viscous processed or unprocessed fuels, and can be scaled to different heat release rates.